We in the conservation business are continually fighting on a daily basis with what is known as the Nine Agents of Deterioration. I thought it would be good to have a series of ‘Spotlight’ blog posts in the coming months to share what the nine agents are, their effects, and what we can do to prevent them taking their toll on Nostell Priory and its collections. You might be surprised to discover that most of the Nine Agents are what you also fight against when doing your own housekeeping!
Agent No. 1 – LIGHT
Light (or radiation) is one of the most obvious enemies of historical objects – everyone has seen things which have faded which were bright and colorful when first made but are now a shadow of their former self. The damaging effects of light come from both ultraviolet (UV) and visible light, and cause disintegration, fading, darkening, and yellowing of the outer layer of organic materials and some coloured inorganic materials. These are usually irreversible changes. We measure visible light in ‘lux levels’, and there are limits for what we consider the maximum level of light that should fall on an object. It is 50 lux for highly light sensitive materials (such as paper, watercolours, wallpaper, carpets), and 200 lux for moderately light sensitive materials (such as oil paintings and stone).
At Nostell there are a number of things that we do to reduce the effects of light attacking our property and the collection inside. Every window has a UV film on, which completely blocks out any harmful UV light coming into the rooms.
We measure the light levels in rooms on a yearly basis. On the two photographs below, there are little squares of card with blue patches – these are called ‘blue wool dosimeters’. They contain pieces of a special grade blue wool, and are left out for one year in places where we especially want to monitor light damage. When the blue wool fades, the amount of fading is measured with a spectrophotometer by a specialist conservator. If the wool is fading too fast, we know to move the object in question, or further reduce the amount of light coming into the room.
We place the dosimeters at varying points of distance from windows – the top photo’s dosimeter is fairly close to a window, and so we would expect some moderate fading. This blue wool directly above, however, is at the far at end of the room away from the windows, and so should hopefully show almost no fading at all over one year
Double blinds are also used to control the light levels. The dark green ones are the ‘blackout blinds’, and are lifted when the house is open to visitors. The pale cream ones are the sun blinds, and they are adjusted throughout the day to compensate for the movement of the sun. This prevents direct sunlight from hitting objects. Ideally, the double blinds should allow less than two per cent of the light falling on them to go into a room.
We’ve also recently taken delivery a conservation frame which focuses on UV light. Half of the frames has a UV-resistant coating, and one half doesn’t. When the beads are moved across the frame, they remain white when under the UV film, but turn pinky red when exposed to UV light in the section of the frame which has no UV coating – a kind of ‘sunburn’, you might say. The photo below shows the effect of UV light on the beads. It’s hoped that by looking at this frame, visitors will understand about our fight against light.